This application claims the priority of German application 198 21 643.2, filed in Germany on May 14, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for piecing a yarn end at a spinning station of an open-end rotor spinning machine, in which process a feed roller, stopped before the piecing operation, is activated for a short pre-feeding period, whereby the end of a sliver, in the form of a fiber beard, is presented to a rotating opening roller for the purpose of renewing the fiber beard, in which process further the single fibers combed from the fiber beard during the feeding period are removed as waste, and in which process the feed roller is re-activated for the actual spinning process. The present invention relates further to an open-end rotor spinning machine for carrying out the process.
In a process of this type described in U.S. Pat. No. 4,059,946, a fiber beard is generated which is uniform in shape and thus permits an exact dosage of fed fibers for the piecing process. This is achieved in that, between the ending of the pre-feeding period and the re-activating of the feed roller for actual piecing, an exact, predetermined further time period is observed. Although single fibers are combed from the fiber beard during this further time period, which single fibers are also removed as waste, still a reproducible fiber beard is created, which comprises a defined amount of fibers and which thus delivers a defined amount of fibers when the feed is re-activated for actual piecing.
The removal of combed single fibers is effected in the case of operating open-end rotor spinning machines usually in that the single fibers to be removed reach the spinning rotor by means of a fiber channel and from there are sucked off over the rotor edge. This functions well as long as the spinning rotor is at a standstill or driven at a very low speed. For piecing, however, as a rule the previously stopped spinning rotor is started up again to reach a piecing speed, during which it can happen that some single fibers, which should be removed as waste, are not suctioned off when the spinning rotor is rotating within a certain speed range, although the centrifugal forces of the spinning rotor while running at a low speed are not really sufficient to hold the single fibers. These "stray" single fibers roll up in the spinning rotor and form so-called fiber nests, which are deposited onto the piecing point of the spun yarn end and thus impair the piecing point in the yarn.
In order to avoid these stray single fibers it is known from the German published patent application 196 24 537, that during the piecing process, a valve is opened on the rotor housing which encases the spinning rotor, through which valve an air stream is blown into the spinning rotor, which supports the action of suctioning off stray single fibers. The air stream is maintained until there are no more stray single fibers left and until the fed single fibers for the actual piecing are held in the spinning rotor by means of centrifugal forces. This procedure, which functions so well in the case of the larger spinning rotors, fails, however, in the case of extremely small spinning rotors having a fiber-guiding diameter of less than 27 mm.
It is an object of the present invention to carry out the piecing process without stray single fibers.
This object has been achieved in that the feed roller is rotated so far in reverse after the pre-feeding period is over that the fiber beard leaves the effective area of the opening roller.
As the opening roller does not comb any more single fibers from the fiber beard after the feed period is over, stray single fibers cannot occur in the first place. Furthermore, the tensile strength of the piecing point is improved by the more even fiber distribution. It is, of course, understood that the feed roller is only rotated in reverse far enough to still permit the fiber beard to be present, as before.
Although it is known from German published patent application 20 18 701 that the feed roller is rotated in reverse by a predetermined distance directly in the case of an end break so that damage to the fibers caused by continued combing of the fiber beard is avoided, and thus that no undesired fibers get into the spinning arrangement, the measures involved in the present invention are not ones to be applied directly in the case of an end break, but are rather to be met in the course of a renewal of a previously combed fiber beard during a piecing process.
In a further advantageous arrangement of preferred embodiments of the present invention, the spinning rotor is stopped until the fiber beard has left the area of effect of the opening roller. This ensures that single fibers combed from the fiber beard, which reach the spinning rotor, are with certainty removed completely as waste. They cannot roll up to form fiber nests in the spinning rotor.
The dosage for actual piecing can be improved when the time delay, corresponding to the reversing of the feed roller, is taken into consideration as a time allowance when the feed roller is activated again for the actual piecing. Thus it is taken into account that the pulled back fiber beard must first reach its actual previous starting position, even when the time allowance only involves fractions of a second.
In order to carry out the process, a traveling piecing device is provided in preferred embodiments according to the present invention, which comprises an auxiliary drive, with which the feed roller can be driven in both rotational directions.